1. Field of the Invention
The invention relates to a computed tomography method which includes the following steps:
generating a conical radiation beam which emanates from a radiation source and traverses an examination zone or an object situated therein,
generating a relative motion in the form of a helix, comprising a rotation about an axis of rotation and a displacement parallel to the axis of rotation, between the radiation source on the one side and the examination zone or the object on the other side, acquiring, during the relative motion and using a detector unit, measured values which are dependent on the intensity in the radiation beam to the other side of the examination zone,
rebinning the measured values so as to form a number of groups of measured data,
filtering the measured data of the groups formed by rebinning, which filtering involves filtering operations performed on different sub-groups of measured data,
reconstructing the absorption in voxels of the examination zone by backprojection of the filtered measured data of different groups.
2. Description of Related Art
A computed tomography method of this kind is known from PCT/IB 99/00027 (PHQ 98-020). Rebinning produces groups of measured data which are associated with rays which would occur if a flat, rectangular detector in a plane containing the axis of rotation were to detect the measured data of a radiation source extending along a helix arc and emitting fan beams perpendicularly to the detector plane and parallel to the axis of rotation. All measured data belonging to parallel fan beams then form a group which is composed of a number of sub-groups. Each sub-group comprises the measured data which are subjected to a common filtering operation and are associated with a horizontal row of the (virtual) detector. Despite an attractive quality of the CT image produced from the filtered measured data by backprojection, such an image may still contain image artefacts, particularly if the conical radiation beam has a large angle of aperture in the direction perpendicular to the axis of rotation and parallel thereto.
It is an object of the present invention to enhance the image quality further in a method of the kind set forth. This object is achieved in that the subdivision into sub-groups is such that the measured data of different sub-groups result from different projections of voxels from at least approximately the same surface within the examination zone.
Each group of measured data comprises the projection of all voxels each time present in the beam path in a given projection direction. A part of these voxels belongs to each sub-group of measured data subjected to a (common) filtering operation. According to the known method the collection of voxels whereto the measured data subjected to a common filtering operation belong changes from one projection to another. The invention is based on the insight that the image artefacts involved in the known method result from such a continuously changing collection. Therefore, according to the invention the filtering operations in the various groups of measured data or for the different projections always involve those measured data which result, at least approximately, from the projection of the same voxels in the object to be examined. The image quality is thus enhanced.
The principle on which the invention is based, i.e. carrying out the filtering operation in such a manner that the filtering operations involve the measured data resulting from different projections of each time the same voxels, is not only valid for the method set forth in the preamble. It can be very advantageously used, however, in its version and in which a rebinning operation is performed so as to form groups of measured data which are associated with rays situated in planes parallel to one another and parallel to the axis of rotation.
When the surface is projected from a different direction, the projection on the detector unit usually will no longer describe a line but rather an elongated surface. Even though only one-dimensional filtering takes place along a (curved) line approximating said surface in a further embodiment, image artefacts are suppressed to a high degree. However, a one-dimensional filtering operation is performed along a straight line which approximates said surface, the image quality thus also being enhanced.
In the preferred embodiment the radiation source and the detector unit describe an angle of exactly 180xc2x0 (viewed from the relevant voxel) around each voxel from its entry into the conical radiation beam until its exit therefrom.
The best image quality can be achieved when first a three-dimensional reconstruction of the absorption distribution in the examination zone is performed and subsequently the absorption distribution within arbitrary two-dimensional layers is extracted therefrom. In another embodiment, however, first a two-dimensional reconstruction takes place in a flat layer which approximates the surface on which the voxels subjected to a common filtering operation are situated. The image quality, however, is not as good as that of the image of the relevant layer when derived from a three-dimensional reconstruction. The method can also be repeated for a plurality of (two-dimensional) layers which are in mutation relative to the axis of rotation and wherefrom a three-dimensional zone can be reconstructed (with a lower image quality). This is known per se from WO 98/448847.
A computed tomograph is also disclosed for carrying out the method according to the invention.